Microfluidics deals with the behavior, precise control and manipulation of small volumes of fluids that are typically constrained to micrometer-length scale channels and to volumes typically in the sub-milliliter range. Here, fluids refer to liquids and either term can be used interchangeably in the rest of the document. In particular, typical volumes of fluids in microfluidics range from 10−15 L to 10−5 L and are transported via microchannels with a typical diameter of 10−7 m to 10−4 m.
At the microscale, the behavior of the fluids can differ from that at a larger, i.e. macroscopic, scale. In particular, surface tension, viscous energy dissipation and fluidic resistance are dominant characteristics of the flow. For example, the Reynolds number, which compares an effect of momentum of a fluid to the effect of viscosity, can decrease to such an extent that the flow behavior of the fluid becomes laminar rather than turbulent.
Fluids do not necessarily mix in the traditional, chaotic sense at the microscale due to the absence of turbulence in low-Reynolds number flows. Interfacial transport of molecules or small particles between adjacent fluids often takes place through diffusion. As a consequence, certain chemical and physical properties of fluids such as concentration, pH, temperature and shear force are deterministic. This provides more uniform chemical reaction conditions and higher grade products in single and multi-step reactions.
A microfluidic probe is a device, in particular a microfabricated scanning device, for depositing, retrieving, transporting, delivering, and/or removing liquids, in particular liquids containing chemical and/or biochemical substances. For example, the microfluidic probe can be used on the fields of diagnostic medicine, pathology, pharmacology and various branches of analytical chemistry. A microfluidic probe can for instance be used for performing molecular biology procedures for enzymatic analysis, deoxyribonucleic acid (DNA) analysis and proteomics.
Retrieving substances from surfaces is important for numerous applications in diagnostics, pharmaceutical and life science research. When substances need to be recovered from different areas, this likely leads to a diffusion of analytes away from their initial recovery volume in the next recovered volume from another area, which potentially causes cross-contaminations between sequentially recovered segments of liquid.